This invention relates to the purification, including drying, of polytetrafluoroethylene of the type prepared by aqueous dispersion polymerization of tetrafluoroethylene.
Polytetrafluoroethylene (PTFE) prepared by aqueous dispersion polymerization in the presence of initiator and surfactant and possibly wax adjuvant is coagulated to form a coagulate of small particles having a diameter on the order of 10-1000 microns. This coagulate, after separation from the bulk of the water, is still contaminated with water, polymerization initiator, surfactant and such other ingredients such as wax, if present. These contaminants have to be removed to the extent that the resultant PTFE fine powder is useful for fabrication into defect-free articles.
Heretofore the PTFE coagulate has been purified by applying it as a shallow bed on the surface of a special paper made of plant fibers and exposing this shallow bed to a flow of heated air, e.g. at least 160.degree. C., through the thickness of the bed, from top to bottom, whereby the contaminants described above volatilize and exit the bed as vapors though openings in the paper at the bottom of the bed, followed by removal of the resultant PTFE fine powder from the surface of the paper. The particles readily separate from the paper surface, leaving behind the paper, itself now contaminated by absorption of at least some of the contaminants, notably surfactant and initiator. Such paper cannot be re-used because of inability to withstand multiple cycles of exposure to the hot air and the problem of the contaminated paper re-contaminating a second batch (bed) of PTFE placed on the paper for purification. Thus, the present process has the problems of cost in continuously replacing the paper support for new batches of PTFE to be freed of contamination and the expense of special precautions being necessary for disposal of the contaminated paper.